1. Field of the Invention
The present invention relates to a link adaptation algorithm, and more particularly, to a congestion-resilient link adaptation method capable of changing a transmission mode through a differentiation between a collision-induced error and a channel-induced error.
2. Description of the Related Art
A number of systems supporting multiple transmission rates to dynamically cope with channel conditions have been proposed. For example, an IEEE 802.11a physical layer (PHY) supports multiple transmission rates with varying a modulation level and a code rate to guarantee throughput performance. An IEEE 802.11a system provides eight PHY modes that are each optimal for different ranges of channel SNR. A procedure of selecting the best among the provided modes is called link adaptation.
One approach to link adaptation is to make a sender monitor channel conditions and determine an optimal mode. However, this approach involves extra implementations or modifications to a current 802.11 standard.
Another approach to link adaptation is to utilize 802.11 acknowledgements. Automatic Rate Fallback (ARF) is a representative example of this approach. According to the ARF scheme, a sender interprets consecutive failures to receive 802.11 acknowledgements (ACKs) as a sign of bad channel conditions. However, the failure to receive the ACKs may be caused by bad channel, collision, or both. Accordingly, the feature of the ARF scheme which does not judge whether the transmission failure is caused by the channel or collision may cause significant performance degradation. For example, if collision is interpreted as bad channel, the link adaptation is unnecessarily triggered, leading to less efficient use of a network.
Recently, an RTS/CTS-based scheme to differentiate between a channel-induced error and a collision-induced error has been proposed. Since RTS/CTS exchange is an optional feature of 802.11, it is undesirable to rely on it. Moreover, overhead of the RTS/CTS exchange itself increases as the link speed increases. Accordingly, this approach is likely to become costly and less efficient.
Accordingly, there is a need for a link adaptation algorithm capable of achieving excellent throughput performance by performing link adaptation through a differentiation between a channel-induced error and a collision-induced error without use of additional hardware or protocol mechanisms.